1. Field of the Invention
The present invention relates to a polishing composition useful for final polishing of the surface of a magnetic disk substrate in the preparation of a substrate for a magnetic disk to be used for a memory hard disk, i.e. a memory device useful for e.g. a computer. More particularly, the present invention relates to a polishing composition to be used for the preparation of a memory hard disk represented by e.g. a Ni--P disk, a Ni--Fe disk, an aluminum disk, a boron carbide disk or a carbon disk, particularly a polishing composition which provides a high stock removal rate in a polishing process for finishing a highly specular surface with a good surface roughness and which, at the same time, is applicable to a production technique for obtaining an excellent finished surface which is useful for a magnetic disk device having a large capacity and a high recording density. Further, the present invention relates to a method for polishing a memory hard disk employing such a polishing composition.
2. Discussion of Background
There have been continuing efforts for miniaturization and larger capacity for memory hard disks to be used for magnetic disk devices which are one of memory media for e.g. computers, and magnetic media are being changed from conventional coating type media to thin film media prepared by sputtering, plating or other methods.
A disk substrate (hereinafter referred to simply as "a substrate") which is most widely used at present, is one having an electroless Ni--P plating film formed on a blank material. Here, the blank material is one obtained by fairing an aluminum or other base plate by lathe processing by diamond turning, lapping by means of a PVA grindstone prepared by fixing SiC grinding material or other methods for the purpose of parallelization or planarization. However, by such various fairing methods, a relatively large waviness can not completely be removed. And, the electroless Ni--P plating film will be formed along the waviness on the blank material. Accordingly, such a waviness will remain also on the substrate, and nodules or large pits will sometimes be formed. Here, the "nodules" are bulges having a diameter of at least about 50 .mu.m, which are formed by bulging of a plating surface at such portions that impurities have been taken into the Ni--P plating film. The "pits" are dents formed by polishing on the surface of the substrate, and "fine pits" are dents having a diameter of less than about 10 .mu.m, among them.
On the other hand, along with the increase in the capacity of memory hard disks, the surface recording density is increasing at a rate of a few tens % per year. Accordingly, the space on a memory hard disk occupied by a predetermined amount of recorded information, is narrower than ever, and the magnetic force required for recording tends to be weak. Accordingly, for recent magnetic disk devices, it is required to minimize the flying height of the head, which is a space between the magnetic head and the memory hard disk, and at present, the flying height of the head is reduced to a level of not higher than 1.0 .mu.inch (about 0.025 .mu.m).
Further, so-called texturing may sometimes be carried out to impart concentric circular scorelines to the substrate after polishing for the purposes of preventing sticking of the magnetic head for reading or writing information, to the memory hard disk and preventing non-uniformity of the magnetic field on the memory hard disk due to scorelines in a certain direction different from the rotational direction of the memory hard disk, formed on the substrate surface by polishing. Recently, for the purpose of further reducing the flying height of the head, light texturing is carried out wherein the scorelines formed on the substrate are further reduced, or a non-texture substrate free from scorelines, is employed which is not subjected to texturing. The technology to support such a low flying height of the magnetic head has also been developed, and the reduction of the flying height of the head is being increasingly advanced.
When a memory hard disk surface has a waviness, the head moves up and down following the waviness of the memory hard disk which is rotated at a very high speed. However, if the waviness exceeds a certain height, the head will no longer be able to follow the waviness, and the head will collide against the substrate surface, thus resulting in so-called "head crush", whereby the magnetic head or the magnetic medium on the memory hard disk surface may be damaged, which may cause a trouble to the magnetic disk device, or which may cause an error in reading or writing information.
On the other hand, head crush may occur also when a micro protrusion of a few .mu.m is present on the memory hard disk surface. Further, when a pit is present on a memory hard disk, it is likely that information can not completely be written in, thus leading to a defect of information so-called a "bit defect" or failure in reading the information, which causes an error.
Accordingly, it is important to minimize the surface roughness of the substrate in the polishing step i.e. the step prior to forming a magnetic medium, and at the same time, it is necessary to completely remove a relatively large waviness as well as micro protrusions, fine pits and other surface defects.
For the above purpose, it used to be common to carry out finishing by one polishing step by means of a polishing composition (hereinafter sometimes referred to as a "slurry" from its nature) comprising aluminum oxide or other various abrasives and water as well as various polishing accelerators. However, by a single polishing step, it has been difficult to satisfy all of the requirements for removing a relatively large waviness as well as surface defects such as nodules and large pits on the substrate surface and for minimizing the surface roughness within a certain period of time. Accordingly, a polishing process comprising two or more steps, has been studied.
In a case where the polishing process comprises two steps, the main purpose of the polishing in the first step will be to remove a relatively large waviness as well as surface defects such as nodules and large pits on the substrate surface, i.e. fairing. Accordingly, a polishing composition is required which has a high ability of correcting the above-mentioned waviness and surface defects without forming deep scratches which can not be removed by polishing in the second step, rather than minimizing the surface roughness.
The purpose of polishing in the second step i.e. finishing or final polishing, is to minimize the surface roughness of the substrate. Accordingly, it is important that the polishing composition is capable of minimizing the surface roughness and capable of preventing formation of micro protrusions, fine pits or other surface defects rather than it has a high ability for correcting a large waviness or surface defects as required for polishing in the first step. Further, from the viewpoint of the productivity, it is also important that the stock removal rate is high. So far as the present inventors are aware, in the conventional two step polishing, it was possible to obtain a substrate surface having a good surface roughness in the polishing in the second step, but the stock removal rate was very low and inadequate for practical production. The degree of the surface roughness is determined depending upon the process for producing the substrate, the final recording capacity as a memory hard disk and other conditions. However, depending upon the desired degree of surface roughness, a polishing process comprising more than two steps, may be employed.
For the above purpose, particularly in finishing polishing in a two step process, it has been common to carry out polishing by means of a polishing composition prepared in such a manner that aluminum oxide or other abrasive is thoroughly pulverized and adjusted for proper particle size, water is added thereto, and aluminum nitrate or various organic acids and other polishing accelerators are incorporated thereto, or a polishing composition comprising colloidal silica and water. However, the polishing by means of the former polishing composition had a problem that the balance between the mechanical component and the chemical component was poor, and micro protrusions or fine pits tended to form. The polishing by means of the latter polishing composition had a problem such that the stock removal rate was so low that it took a long time for polishing, and the productivity was low, roll off (or "dub off") as an index of sagging of an end face of the substrate tended to deteriorate, or washing after the polishing tended to be difficult.
In order to solve the above problems, it has been proposed to employ a polishing composition having various additives for accelerating the processing added to colloidal silica, in final polishing of a substrate to be used for a memory hard disk. For example, JP-A-9-204657 discloses a polishing composition having aluminum nitrate and a stabilizer incorporated to colloidal silica, JP-A-10-204416 discloses a polishing composition having an iron compound incorporated to colloidal silica, and JP-A-11-167714 proposes a polishing composition having hydrogen peroxide incorporated to colloidal silica. Further, it has been proposed to use fumed silica in JP-A-9-208934, titanium oxide in JP-A-10-121035 and zirconium oxide in JP-A-10-121034, instead of aluminum oxide as the conventional abrasive for polishing compositions. These proposals are directed to present polishing compositions capable of providing a polished surface with small surface roughness and little microprotrusions, micropits or other surface defects, as used to be desired for polishing compositions.
The present inventors have tested these compositions and have confirmed that these conventional polishing compositions are effective to reduce the surface roughness and surface defects as intended, although the effectiveness varies among the compositions. However, at the same time, it has been found that these compositions have a problem such that when they are used for polishing substrates by means of a double side polishing machine, chattering or carrier noise of a carrier for holding such a substrate, is large, and in an extreme case, a chamfer portion which is a peripheral face of the substrate, is likely to be damaged by collision of the substrate and the carrier.
When a double side polishing machine is used for polishing the surface of a magnetic disk substrate, the substrate is supported by a carrier. The carrier is held between a planetary gear (internal gear) along the outer periphery of the polishing machine and a sun gear at the center of the polishing machine. At the time of polishing, the substrate is acted on and polished via the gear and the carrier. At that time, if the friction between the substrate and the carrier at a clearance (a so-called play which is not required for transmission of the action) provided between the carrier and the gear of the polishing machine, is non-uniform in the interior of the polishing machine, the substrate and the carrier respectively undergo vibration, whereby a vibration noise which is a so-called chattering or carrier noise will be generated as a whole. If the chattering is caused by the vibration of the substrate in the carrier, in an extreme case, the outer periphery of the substrate will collide with the inner periphery of the carrier, thus leading to a damage of the chamfer, as mentioned above.